1. Technical Field
The present invention relates to an automatic screw tightening apparatus, and in particular, to an automatic screw tightening apparatus that transfers a screw from a screw aligning/supplying apparatus to a screw tightening driver unit by, for example, a transfer tube, and tightens the screw against a screwed subject.
2. Related Art
Well-known general screw tightening apparatuses adapt, as a method of supplying a screw to a fore end of a driver bit, (1) a method of directly picking a screw up from a screw aligning/supplying apparatus with a driver, (2) a method in which a screw transfer tube is brought into close contact with a fore end of a driver bit and a screw is directly sucked to a driver, and (3) a method of transferring a screw from a screw aligning/supplying apparatus to a driver unit by a pressure of compressed air.
The above-mentioned first screw supplying method is, for example, a method of aligning screws in one line by a screw aligning/supplying apparatus, moving a driver to the upper side of a screw positioned at a predetermined location, lowering the driver to the location of the screw, and drawing the screw by a screw sucking device installed in the driver such that the screw is engaged with the bit of the driver, as disclosed in Japanese Patent Laid Open Publication No. Hei9-58847.
The above-mentioned second screw supplying method is a method that brings a screw transfer tube d into direct contact with a fore-end part of a mouthpiece f provided outside of a driver bit e, directly draws a screw b into a mouthpiece f by sucking the screw b aligned by a screw aligning/supplying apparatus (not shown) by a screw sucking device installed in a driver c, moves the driver bit e, and tightens the screw b against a screwed subject part, as shown in FIG. 16.
The above-mentioned third screw supplying method is, for example, a method of supplying a screw b1 aligned by a screw aligning/supplying apparatus from a compressed-air supplying unit of the screw aligning/supplying apparatus to a fore-end part c1 of a driver c through a screw transfer unit d such as a tube by a pressure of compressed air, as shown in (a) and (b) of FIG. 17.
In this case, while the screw b1 is tightened against a screwed subject part by a driver bit e, the next screw b2 is supplied to the driver c as shown in (a) of FIG. 17. Further, when tightening of the previous screw b1 is completed and lifting of the driver bit e is completed, the next screw b2 moves to the fore-end part c1 of the driver c, and when the driver bit 3 is lowered down again, the screw b2 is engaged with the driver bit e ((b) of FIG. 17). Here, the fore-end part c1 of the driver c is configured to be openable and closable so that when the screw b2 is tightened to a screwed subject part g, the fore-end part c1 is opened, and when the next screw b2 is supplied after the tightening of the screw is completed, the fore-end part c1 is closed, whereby screws b are automatically and consecutively supplied.
However, in the above-mentioned first and second screw supplying methods, since the driver c should move to a predetermined position of the screw aligning/supplying apparatus whenever tightening of each screw b against a screwed subject part is completed, there are disadvantages including the followings: a time interval from a time point when tightening of one screw b is completed to a time point when tightening of the next screw starts becomes long; a lot of time is taken to tighten all screws b against screwed subject parts; and it is required to closely position the screw aligning/supplying apparatus and the driver c and to accurately determine the distance between the screw aligning/supplying apparatus and the driver c.
Further, in the second screw supplying method, since a screw b is directly sucked from the screw aligning/supplying apparatus by the screw sucking device installed in the driver c, the structure of the screw aligning/supplying apparatus and the fore-end part of the driver c can be simplified. However, since a screw b is directly sucked into the mouthpiece f, while one previous screw b is tightened, the next screw b cannot be supplied. Therefore, there is a disadvantage in which an amount of time required to tighten all screws to screwed subject parts becomes large, similar to the first screw supplying method.
Furthermore, in general, whether a screw b has reached the inside of the mouthpiece f is detected by a pressure detector of a screw sucking device. However, there are disadvantages in which accuracy of detecting a variation in an air pressure is low and accuracy of detecting that a screw b has reached is low.
In the third screw supplying method, unlike the first and second screw supplying methods, it is unnecessary to move the driver c whenever a screw b is tightened to a screwed subject part, and while a screw b is tightened, the next screw b1 is prepared. Therefore, an amount of time required to tighten all screws to screwed subject parts is remarkably reduced as compared to the first and second screw supplying methods.
However, in a case of the third screw supplying method, when a screw b once supplied to the driver c moves to and is engaged with a fore-end of the driver bit e, the screw is transferred with the tip of the thread part of the screw b being in the lead and with the head part of the screw at the tail end. Therefore, there is a disadvantage in which the position of the screw b becomes unstable. In particular, when the length of the screw b is short, the direction of the screw b may be reversed or get stuck on its way. Therefore, there is a disadvantage in which it is difficult to stably supply screws b to the driver bit e.
Moreover, since the screw b is transferred with the tip of the thread part of the screw b being in the lead, there are disadvantages in which the screw may damage the inner wall of a transfer tube or get stuck in the transfer tube.